:composed_of conversion
====================

UPDATE: This plugin was added to Rails 2.0 (http://dev.rubyonrails.org/changeset/8003)

<tt>composed_of</tt> is handy, but it can also be a real pain.  Primarily in that it makes it hard to use :composed_of fields in Rails forms.  This rewrite of the Active Record :composed_of method allows you to specify a block to convert incoming parameters to the correct object type.

This has been most helpful when using the Money gem:

  class Account < ActiveRecord::Base
    composed_of :balance, :class_name => "Money", :mapping => %w(cents cents) do |amount|
  	  amount.to_money
    end
  end
  
I Account#balance can now can have a form of type casting like any other attribute:

  >> account = Account.new :balance => 100
  >> account.balance
  => #<Money:0x2612770 @cents=10000, @currency="USD">
  
And now it can transparently be used in forms:

  <%= text_field :account, :balance %>

This approach can even be used for more advanced aggregations:

  class User < ActiveRecord::Base
    composed_of :address, :class_name => "Address"
  		:mapping => [%w(street street), %w(city city), %w(state state), %w(zip zip)] do |addr|
  	Address.new(addr[:street], addr[:city], addr[:state], addr[:zip])
    end
  end

A user can now be created from a hash:

  User.new(:address => {:street => "123 A Street", :city => "Somewhere", :state => "NO", :zip => 12345})

Additionally, the current version of <tt>composed_of</tt> builds a string and evals it to define the attribute accessors.  It's dirty.  This rewrite was submitted as a patch to the Rails trac (http://dev.rubyonrails.org/ticket/6322) but is yet to be accepted.